1. Field of the Invention
This invention relates to a process for preparing a bolometer material having excellent properties which is used in a temperature-measuring apparatus in which a sensor device is formed using a material utilizing thermal variation of resistance, or a non-cooling type of two-dimensional infrared imaging apparatus not requiring a cooling device, in which a material utilizing thermal variation of resistance is formed as an element and the element is two-dimensionally aligned. This invention also relates to a bolometer device formed by the process, in particular a process for preparing a protective film which does not deteriorate the above excellent properties.
2. Description of the Prior Art
Generally, a metal oxide with relatively large thermal variation of resistance (or TCR: Temperature Coefficient of Resistance) is used as a material suitable to a bolometer. In particular, vanadium oxide VOx is known to easily provide a higher TCR value. For applying VOx as a sensor in a non-cooling type of two-dimensional infrared imaging apparatus, it must have a thin-film form. A VOx thin-film is generally formed by physical vapor deposition such as vacuum deposition and sputtering (An example of preparation by sputtering has been disclosed in Jerominek et al., Optical Engineering, 32 (9), p. 2093 (paragraph 2) (1993)).
Furthermore, for using VOx as a bolometer device, a higher TCR is, of course, required to achieve higher sensitivity and a specific resistance of VOx (or synonymically xe2x80x9cresistivityxe2x80x9d) must be controlled to a value necessary for configuring a circuit. For example, for achieving such controlling, Wada et al. has disclosed in JP-A 9-145481 a process for providing required specific resistance and TCR value with a heating temperature during a VOx thin-film is heated under a reducing atmosphere containing hydrogen.
In such a process for controlling TCR and specific resistance with a heating temperature, as illustrated in FIG. 6, a VOx film is deposited and a crystal phase of V2O5 (x=2.5) is formed. Then, it is heated and reduced under an atmosphere containing hydrogen. Table 1 shows properties obtained by such a prior art procedure. FIG. 1 shows a relationship between TCR and specific resistance obtained by the prior art. As seen from FIG. 1, in this prior art, a TCR of xe2x88x922%/K provides VOx (x is approximately 2) having a specific resistance of about 0.3 xcexa9cm (3xc3x9710xe2x88x923 xcexa9m). In the prior art, a material such VOx property (x is approximately 2) has been used for forming a device.
When forming a device using a material such properties, an insulating film such as a silicon oxide (SiO2) film and a silicon nitride (SiN) film is generally formed as a protective film on the surface of VOx by chemical vapor deposition (CVD) for preventing deterioration in the properties.
According to a conventional process, a lower heating temperature must be employed for providing VOx with higher sensitivity (higher TCR). However, a lower heating temperature increases a TCR while increasing a specific resistance.
As a specific resistance is increased, charged carriers in the VOx thin-film are reduced, leading to increase of a so-called 1/f noise (f is a frequency), i.e., a frequency noise proportional to an inverse of the number of charged carriers. Therefore, in the prior art process, noise increase is prevented by increasing a volume of VOx for increasing the number of carriers, i.e., increasing a film thickness.
Increase in a volume is, however, not preferable for processing the material as a device. For example, a thicker film may lead to a longer heating time. Furthermore, it may lead to a larger step on a side after processing, i.e., poor flatness, which may cause the problem in process working that a metal interconnection even with a thickness of about 0.1 xcexcm may be broken.
Additionally, when forming a protective film in a bolometer by CVD, a substrate is generally heated to a temperature of 200xc2x0 C. or higher and then very reactive gases such as silan (SiH4), ammonia (NH3) and/or dinitrogen monoxide (N2O) are used, which may react with VOx to be protected to change VOx properties.
Objectives of this invention are to improve quality of a VOx film having a specific resistance such that 1/f noise is reduced while giving higher sensitivity (higher TCR) than the prior art and to provide processes for preparing a bolometer material and for forming a protective film which allow us to coat the VOx surface with a protective film while maintaining the property.
In this invention, crystal phase of VOx prepared is V2O3 (x=1.5), i.e., a starting film is made of a material having a specific resistance lower than a desired value for a material used as a bolometer. The crystal phase is heated under an oxidizing atmosphere containing oxygen for modifying the crystal phase to that with the desired specific resistance. In this thermal processing, the film property is controlled by adjusting not a temperature, but a thermal oxidation time.
Specifically, this invention provides a process for preparing a material for a bolometer using a vanadium oxide VOx film, wherein crystal phase comprising V2O3 (x=1.5) is used as a starting film material and the crystal phase is heated under an oxidizing atmosphere containing oxygen.
In this invention, the VOx surface is coated with a protective film such as a silicon nitride film SiNxxe2x80x2or a silicon oxynitride film SiOxxe2x80x3Ny by physical deposition such as sputtering for protecting the improved VOx property to provide a bolometer device.
The preparation process for a bolometer material of this invention may be used to improve the bolometer material property in comparison with that according to the prior art, and thus to provide a high-performance infrared sensor device with higher sensitivity and reduced 1/f noise of the bolometer material without deterioration in the property. Furthermore, since a film thickness may be reduced compared with that in the prior art, processing for forming a device may be more easily conducted.
Additionally, according to a process for forming a protective film of this invention, a protective film can be formed without deterioration in the excellent properties of the VOx film.